as for controllable aircraft, i have no doubt its possible. to buy them? definitely. however, i dont think you can build them. will EF or any other competitive mod do this? definitely not. the game would be imbalance beyond control. there are only a few AA units in the game, making it overpowered.
balance. if one could continually call in infinite arty pieces untill there was no pop left, it would not make for very fun and competitive gameplay.